The present invention relates to integrated circuits incorporating resistors.
A problem in the prior art of resistors for use in integrated circuits has been the temperature dependence of the resistance. Polysilicon resistors normally have a very large negative temperature coefficient of resistance (TCR), whereas diffused resistors in monocrystalline silicon typically have a very large positive TCR. For many device applications, it is highly desirable to have resistors with small or zero TCR.
Thus, it is an object of the present invention to provide a method for fabricating integrated circuits having resistors with zero or small variation in resistance with temperature change. Resistors having near-zero TCR are desirable for various applications, such as analog circuits, data conversion circuits and specialty instrumentation circuits, and static random access memories (SRAMs).
However, a problem which affects a larger range of devices is the thermal instability which can result from resistors having a negative TCR. That is, a resistor having a negative TCR, if temporarily overheated, will pass more current, and thus will be likely to receive even more ohmic heating. In some circuits, this can cause thermal runaway which destroys the resistor and/or other parts of the circuit. This is a particular problem in the art of bipolar devices. For this reason, bipolar devices have typically used diffused resistors because of their positive TCR. However, the use of diffused resistors makes device layout much more difficult and results in a more complex processing.
Thus, it is an object of the present invention to provide a method for fabricating polysilicon resistors whose resistance increases with increasing temperature.
It is a further object of the present invention to provide bipolar integrated circuits incorporating polysilicon resistors whose resistance increases with temperature.
The problems of thermal instability and resistors are particularly important in bipolar integrated circuits commonly due to the relatively high current densities commonly used, but are not by any means limited to that art.
Thus, it is an object of the present invention to provide integrated circuits incorporating polysilicon resistors whose resistance increases with temperature.
Another problem with integrated circuit resistors according to the prior art is their radiation hardness. That is, conventional resistors will change their TCR significantly when irradiated, because radiation-generated carriers are trapped at defect sites or ground boundaries in the resistor.
Thus, it is an object of the present invention to provide integrated circuits incorporating resistors whose TCR does not change substantially with radiation.
According to the present invention there is provided:
A method for fabricating polysilicon resistors in integrated circuits, comprising the steps of:
depositing polysilicon;
transiently annealing said deposited polysilicon to increase the grain size thereof;
passivating said transiently annealed polysilicon with a passivating species, to reduce the trap density at the grain boundaries of said polysilicon; and
forming connections to said polysilicon to form resistors.